Phenol and substituted phenols are important products in the chemical industry and are useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene relative to that of butenes is likely to increase, due to a developing shortage of propylene.
Thus, a process that uses butenes or higher alkenes instead of propylene as feed and coproduces methyl ethyl ketone (MEK) or higher ketones, such as cyclohexanone, rather than acetone may be an attractive alternative route to the production of phenols. For example, there is a growing market for MEK, which is useful as a lacquer, a solvent and for dewaxing of lubricating oils. In addition, cyclohexanone is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam and nylon 6.
It is known that phenol and MEK can be produced from sec-butylbenzene, in a process where sec-butylbenzene is oxidized to obtain sec-butylbenzene hydroperoxide and the peroxide decomposed to the desired phenol and methyl ethyl ketone. An overview of such a process is described in pages 113-121 and 261-263 of Process Economics Report No. 22B entitled “Phenol”, published by the Stanford Research Institute in December 1977.
However, in comparison to cumene, oxidation of aromatic compounds substituted by branched alkyl groups having 4 or more carbon atoms, such as sec-butylbenzene, to the corresponding hydroperoxide requires higher temperatures and is very sensitive to the presence of impurities. For example, in the case of sec-butylbenzene containing 1% by weight of isobutylbenzene, the rate of formation of sec-butylbenzene hydroperoxide decreases to about 91% of that when the sec-butylbenzene is free of isobutylbenzene. Similarly, when the isobutylbenzene content is 1.65% by weight, the rate of oxidation decreases to about 86%; when the isobutylbenzene content is 2% by weight, the rate of oxidation decreases to about 84%; and when the isobutylbenzene content is 3.5% by weight, the rate of oxidation decreases to as low as about 82%.
Thus there remains a need to find an oxidation process for producing for example C4+ alkyl aromatic hydroperoxides, and particularly sec-butylbenzene hydroperoxide, that is much less sensitive to the presence of impurities than the existing oxidation processes, and that allows efficient commercial scale production of phenol and MEK or higher ketones.
U.S. Pat. Nos. 6,852,893 (Creavis) and 6,720,462 (Creavis) describe methods for producing phenol by catalytic oxidation of alkylaromatic hydrocarbons to the corresponding hydroperoxide, and subsequent cleavage of the hydroperoxide to give phenol and a ketone. Catalytic oxidation takes place with oxygen, in the presence of a free radical initiator and a catalyst, typically an N-hydroxycarbodiimide catalyst, such as N-hydroxyphthalimide. Preferred substrates that may be oxidized by this process include cumene, cyclohexylbenzene, cyclododecylbenzene and sec-butylbenzene.
In addition, the article by Sheldon et al entitled “Organocatalytic Oxidations Mediated by Nitroxyl Radicals” in Adv. Synth. Catal., 2004, 346, pages 1051-1071 discloses that cyclohexylbenzene (CHB) can be oxidized to the 1-hydroperoxide with 97.6% selectivity at 32% CHB conversion at 100° C. in the presence of 0.5 mol % of a N-hydroxyphthalimide catalyst and 2 mol % of the product hydroperoxide as a free radical initiator.
However, while N-hydroxycarbodiimides have shown activity and selectivity for the oxidation of alkylaromatic compounds to their corresponding hydroperoxides, they suffer from the problems inherent in any homogeneous catalyst in that they are tend to be removed from the reaction zone with the product effluent and so must be separated from the product effluent. There is therefore significant interest in developing a heterogeneous oxidation catalyst for producing alkylaromatic hydroperoxides provided adequate activity and selectivity can still be maintained.
In our International Patent Publication No. WO 06/15826 we have described a process for producing phenol and methyl ethyl ketone, in which a feed comprising benzene and a C4 alkylating agent is contacted under alkylation conditions with catalyst comprising zeolite beta or a molecular sieve having an X-ray diffraction pattern including d-spacing maxima at 12.4±0.25, 6.9±0.15, 3.57±0.07 and 3.42±0.07 Angstrom to produce an alkylation effluent comprising sec-butylbenzene. The sec-butylbenzene is then oxidized to produce a hydroperoxide and the hydroperoxide is cleaved to produce the desired phenol and methyl ethyl ketone. Although homogeneous catalysts, such as N-hydroxy substituted cyclic imides, are disclosed as suitable for the oxidation reaction, preferred catalysts are said to be heterogeneous catalysts, such as the oxo (hydroxo) bridged tetranuclear manganese complexes described in U.S. Pat. No. 5,183,945 and U.S. Pat. No. 5,922,920.
Manganese oxide octahedral molecular sieves possessing mono-directional tunnel structures are also known and constitute a family of molecular sieves wherein chains of MnO6 octahedra share edges to form tunnel structures of varying size. Such materials have been detected in samples of terrestrial origin and are also found in manganese nodules recovered from the ocean floor. Manganese nodules have been described as useful catalysts in the oxidation of carbon monoxide, methane and butane (U.S. Pat. No. 3,214,236 [Mobil]), in the reduction of nitric oxide with ammonia (Atmospheric Environment, Vol. 6, p. 309 (1972)) and in the demetallation of topped crude in the presence of hydrogen (Ind. Eng. Chem. Proc. Dev., Vol. 13, p. 315 (1974)).
The hollandites are naturally occurring hydrous manganese oxides with tunnel structures consisting of double chains of MnO6 octahedra which share edges to form (2×2) tunnel structures. The average size of these tunnels is about 4.6 A° square. Ba, K, Na and Pb ions are present in the tunnels and coordinated to the oxygens of the double chains. The identity of the tunnel cations determines the mineral species. Specific hollandite species include hollandite (BaMn8O16), cryptomelane (KMn8O16), manjiroite (NaMn8O16) and coronadite (PbMn8O16). A hydrothermal method of synthesizing manganese oxide octahedral molecular sieves possessing (2×2) tunnel structures such as those possessed by the naturally-occurring hollandites is described in “Hydrothermal Synthesis of Manganese Oxides with Tunnel Structures,” in Synthesis of Microporous Materials, Vol. II, 333, M. L. Occelli, H. E. Robson Eds. Van Nostrand Reinhold, N.Y., 1992. Such synthetic octahedral molecular sieves having (2×2) tunnel structures are referred to in the art by the designation OMS-2.
The todorokites are naturally occurring manganese oxides with (3×3) tunnel structures formed by triple chains of MnO6 edge-sharing octahedra. Todorokites and related species are described by Turner et al. in “Todorokites: A New Family of Naturally Occurring Manganese Oxides”, Science, Vol. 212, pp. 1024-1026 (1981). The authors speculate that since todorokites are often found in deep-sea manganese nodules containing high concentrations of copper and nickel, it is probable that such metals substitute for Mn2+ in the octahedral framework. Todorokites have attracted particular interest because of their relatively large tunnel dimensions and their cation-exchange behaviour which is similar to that of zeolites (Shen et al., “Manganese Oxide Octahedral Molecular Sieves: Preparation, Characterization, and Applications”, Science, Vol. 260, pp. 511-515 (1993)). A method of synthesizing a manganese oxide octahedral molecular sieve possessing (3×3) tunnel structures such as those possessed by the naturally-occurring todorokites is described in U.S. Pat. No. 5,340,562 (Texaco). Such synthetic octahedral molecular sieves having (3×3) tunnel structures are referred to in the art by the designation OMS-1.
Synthetic manganese oxide octahedral molecular sieves of the OMS-1 and OMS-2 type have been employed as acid-base catalysts in a variety of acid-base organic conversion reactions, such as the conversion of alcohols to ketones and olefins. Manganese oxide molecular sieves having a 4×4 tunnel structure, designated OMS-3, have also been synthesized and have been proposed as CO oxidation catalysts, for example in EP-A-0710622 (Texaco).
According to the invention, it has now been found that manganese oxide molecular sieves show activity and selectivity as catalysts in the oxidation of secondary alkyl substituted benzenes, including sec-butylbenzene and cyclohexylbenzene, to the corresponding hydroperoxides.